1. Field of Invention
This invention relates generally to a process for recycling mixed waste and more particularly to a process for maximizing the recycling of a heterogeneous hazardous waste stream having an uncontrolled, fluctuating content of carbon, metal, and minerals into separate, non-hazardous recycled components.
2. Description of Related Art
There is an ongoing need to dispose of hazardous waste generated by industries, most particularly the chemical industry. Hazardous waste is often either buried or burned, either of which can be costly processes, significantly increasing production costs for the products produced by the relevant industry. The costs for the disposal of hazardous waste typically, in part reflect the excise taxes and fees which must be paid to legally dispose of the waste. However, such hazardous waste disposal excise taxes and fees may be reduced or avoided totally by recycling the hazardous waste into commercial grade chemicals and materials, thereby decreasing the overall costs associated with disposal of the waste.
Gasification is one method of disposing of hazardous waste materials. Typically, the gasification process involves the step of xe2x80x9cpyrolysisxe2x80x9d, which involves heating the waste material to a temperature wherein any water, hydrocarbons, and organic compounds are volatilized and the remaining mineral and metallic constituents are melted into a molten slag. After cooling and solidifying, the molten slag may either be disposed of or utilized in the production of steel. The volatilized hydrocarbons and organic compounds are generally disposed of by burning, and may in fact be consumed as an energy source. However, under current regulations, energy recovery of this sort from hazardous waste is still classified as disposal rather than recycling, thereby still incurring the full amount of taxes and fees associated with disposing of hazardous waste.
However, rather than burning the hydrocarbons and organic compounds, if the oxygen concentration present during the gasification process is controlled, it is possible to partially oxidize the vaporized hydrocarbons and organic compounds producing a xe2x80x9csynthesis gasxe2x80x9d which may be further processed. Synthesis gas typically includes substantial quantities of hydrogen (H2) and carbon monoxide (CO), accompanied by lesser quantities of carbon dioxide (CO2) and water (H2O). Synthesis gas is a raw material suitable for the production of a number of commercial grade chemicals such as, for example and not limitation, ammonia, methanol, and dimethyl ether. Since the use of a synthesis gas to generate commercial products is classified as recycling under current regulations, the excise taxes and fees associated with hazardous waste disposal can be avoided by recycling the synthesis gas in this manner.
Methanol and dimethyl ether are both typically produced from synthesis gas on an industrial scale by a process involving the catalytic conversion of carbon monoxide and hydrogen. Methanol is produced from synthesis gas in the presence of a methanol synthesis catalyst by the reaction (2H2+COxe2x86x92CH3OH). Dimethyl ether is produced by the dehydration of methanol in the presence of a methanol dehydration catalyst by the reaction (2CH3OHxe2x86x92H2O+CH3OCH3). Accordingly, it is often desirable to co-synthesize methanol and dimethyl ether in a reactor containing both a methanol synthesis catalyst and a methanol dehydration catalyst.
Conventional methods of forming methanol require careful balancing of the ratio of H2 to CO present in the synthesis gas during the catalytic synthesis of methanol to approximately 2:1. An excess of carbon monoxide in the synthesis gas will result undesirable levels of carbon dioxide and carbon in the reactor, creating an exothermic event that overheats and ruins the catalyst. Conversely, an excess of hydrogen produces undesirable amounts of waste water during the methanol synthesis reaction which results in economically unfeasible treatment and purification costs. Accordingly, careful control of the composition and flow rate of the feedstock used to produce the synthesis gas is necessary for production of methanol or methanol and dimethyl ether.
For example, in one conventional process, coal is gasified using a strictly controlled feed rate of oxygen, in order to obtain a synthesis gas having a uniform composition and at a uniform rate. In another conventional process, methane is converted into a synthesis gas in a reaction with a precisely controlled amount of steam to produce a synthesis gas having a uniform composition at a uniform rate. In each of these cases, the feed material from which the synthesis gas is produced has a uniform composition, thereby allowing narrow control of the ratio of H2 to CO in the synthesis gas.
Unfortunately, most industrial and hazardous wastes do not contain a uniform mixture of materials. Workers commonly throw a variety of undesirable items into the waste receptacles. Additionally, hazardous waste can contaminate the containers within which it is stored and transported, creating additional waste. Accordingly, the use of heterogeneous industrial and hazardous waste in a conventional gasification processes will result in a synthesis gas having a widely varying ratio of H2 to CO. Thus, it has generally been thought that such heterogeneous industrial and hazardous wastes are unsuitable for use in the production of a synthesis gas suitable for the production of methanol and dimethyl ether.
Accordingly, it is an object of the present invention to provide a process for maximizing the recycling of heterogeneous waste, such as municipal solid waste, industrial waste and hazardous chemical waste into a plurality of commercial grade products and chemical compounds, thereby realizing economic gains from the resale of the commercial grade products avoiding excise taxes and fees associated with disposal of the waste.
Furthermore, it is an object of the present invention to provide a process for converting heterogeneous waste comprising a large number of miscellaneous, unidentified substances into a plurality of product streams having known compositions.
It is yet another object of the present invention to provide a system for converting heterogeneous carbon-containing waste into a synthesis gas having a desired composition suitable for the synthesis of methanol and/or dimethyl ether.
The above objectives are accomplished according to the present invention by providing a process for recycling heterogeneous waste including the initial step of subjecting the heterogeneous waste to pyrolysis to produce a synthesis gas stream comprising at least carbon monoxide and hydrogen and to produce a molten pyrolysis product stream having a variable composition comprising at least a mineral material and a metallic material. The molten pyrolysis product stream is converted to a plurality of commercial grade solid materials. Likewise, the synthesis gas stream is also converted into at least one commercial grade chemical.